Micro-electro-mechanical system having movable element integrated into substrate-based package

ABSTRACT

Semiconductor-centered MEMS ( 100 ) integrates the movable MEMS parts, such as mechanical elements, flexible membranes, and sensors, with the low-cost device package, and leaving only the electronics and signal-processing parts in the integrated circuitry of the semiconductor chip. The package is substrate-based and has an opening through the thickness of the substrate. Substrate materials include polymer tapes with attached metal foil, and polymer-based and ceramic-based multi-metal-layer dielectric composites with attached metal foil. The movable part is formed from the metal foil attached to a substrate surface and extends at least partially across the opening. The chip is flip-assembled to span at least partially across the membrane, and is separated from the membrane by a gap.

This application is related to and claims the benefit of ProvisionalApplication Nos. 61/291,767 filed Dec. 31, 2009 and 61/152,607 filedFeb. 13, 2009.

FIELD OF THE INVENTION

The present invention is related in general to the field ofsemiconductor devices and processes, and more specifically to thestructure and fabrication method of Micro-Electro-Mechanical systems(MEMS) having the movable element integrated into a substrate-based ballgrid array package and the sensing element built on the integratedcircuit.

DESCRIPTION OF RELATED ART

The wide variety of products collectively calledMicro-Electro-Mechanical systems (MEMS) are small, low weight devices onthe micrometer to millimeter scale produced on the basis of batchfabrication techniques similar to those used for semiconductormicroelectronics devices. MEMS integrate mechanical elements, sensors,actuators, and electronics on a common carrier. MEMS have been developedto sense mechanical, thermal, chemical, radiant, magnetic, andbiological quantities and inputs, and produce signals as outputs.

MEMS may have parts moving mechanically under the influence of an energyflow (acoustic, thermal, or optical), a temperature or voltagedifference, or an external force or torque. Certain MEMS with amembrane, plate or beam can be used as a pressure sensor or actuator(for instance microphone and speaker), inertial sensor (for instanceaccelerometer), or capacitive sensor (for instance strain gauge and RFswitch); other MEMS operate as movement sensors for displacement ortilt; bimetal membranes work as temperature sensors. Besides small size,the general requirements for the membrane- or plate-operated sensorsinclude long term stability, small temperature sensitivity, lowhysteresis for pressure and temperature, resistance to corrosiveenvironments, and often hermeticity.

In a MEMS, the mechanically moving parts are fabricated together withthe sensors and actuators in the process flow of the electronicintegrated circuit (IC) on a semiconductor chip. As an example, themechanically moving parts may be produced by an undercutting etch atsome step during the IC fabrication. Bulk micromachining processesemployed in MEMS sensor production for creating, in bulk semiconductorcrystals, the movable elements and the cavities for their movementsinclude anisotropic wet etching, reactive ion etching (RIE), and deepreactive ion etching (DRIE). These techniques employ photolithographicmasking, are dependent on crystalline orientation, and need etch stops,all of which are expensive in terms of time and throughput. In addition,there are bulk and surface micromachining techniques for building upstructures in thin films on the surface of semiconductor wafers, alsoexpensive techniques. While many of the processes are expensive toimplement, some processes, such as automatic wafer bonding, areinexpensive.

Because of the moving and sensitive parts, MEMS have a need for physicaland atmospheric protection. Consequently, MEMS are surrounded by ahousing or package, which has to shield the MEMS against ambient andelectrical disturbances, and against stress. For many devices, fullyhermetic and even quasi-hermetic packages represent a significant costadder, especially when ceramic packages or precision parts such as glassplates are required.

Among the basic operating principles of pressure sensors arepiezoresistive, capacitive, and resonant operation. In thepiezoresistive operation, the pressure is converted to an electronicallydetectable signal, wherein the conversion relies on the elasticdeformation of a structure such as a membrane exposed to the pressure;pressure causes strain, and strain causes change of electricalresistivity. In MEMS silicon technology, controlling the membranethickness, size, and alignment involves precision process steps. In theresonant operation, the pressure causes mechanical stress in thevibrating microstructure; the resonance frequency is measured independence on the mechanical stress. Excitation and damping of the MEMSsilicon diaphragm and the nonlinear frequency-pressure relationshiprequire sophisticated calibration. In the capacitive operation, thepressure causes a displacement-dependent output signal. The change inpressure causes a displacement, the displacement causes a capacitorchange, and the capacitor change causes electrical signal—similar theoperation of a condenser microphone. Nonlinearity and parasiticcapacitances and residual membrane stress represent challenges for MEMSmembrane fabrication of silicon and epitaxial silicon.

Taking the example of capacitive pressure sensors, several fabricationmethods may be chosen. In one method, the sensors are bulkmicro-machined as a glass-silicon-glass structure with verticalfeed-throughs. In another method, a preferentially etched wafer receivesdeep and shallow boron diffusions and dielectric depositions, which aremounted on glass so that the wafer can finally be dissolved. In yetanother method, a surface micro-machined capacitive pressure sensor iscreated by a polysilicon layer (1.5 μm thick) separated by a gap (0.8 μmwide) over the n+doped silicon electrode; the sensor is monolithicallyintegrated with the sensing circuitry. The sensors are small and span anoperating range from about 1 bar to 350 bar, have high overpressurestability, low temperature dependence and low power consumption.

In the basic operating principle of accelerometers, the mechanical andelectrical sensitivity are a function of the vertical displacement ofthe movable plate's center. In displacement sensing accelerometers, theapplied acceleration as input is transformed into the displacement ofthe movable mass (plate) as output; a suspension beam serves as theelastic spring. Force sensing accelerometers detect directly the forceapplied on a proof mass. The MEMS fabrication in bulk single-crystalsilicon of the movable plate, the suspension beam, and the proof massrequires sensitive semiconductor etching techniques.

SUMMARY OF THE INVENTION

Applicants believe manufacturing cost is the dominant factor preventingthe widespread integration of pressure sensors, microphones,accelerometers and other applications in which a movable member isneeded to convert an external analog input into an electrical output,into systems in the automotive, medical, and aerospace industries.

Applicants saw that MEMS built on the surface or within the wafer bystandard wafer fab technology and standard wafer fab lithographicmethods is not only a high cost approach, but also limits the choice ofmaterials and configuration available to the MEMS component, which haveto be compatible with the standard wafer process. After the waferfabrication, in standard technology the MEMS still have to be packagedusing known packaging material and processes—another cost adder.

Applicants solved the problem of mass-producing low costsemiconductor-centered MEMS by integrating the movable MEMS parts, suchas mechanical elements and sensors, including their complete fabricationwith low-cost device materials and packages, and by leaving only theelectronics and signal-processing parts in the integrated circuitry. Thepackage, into which the movable parts are integrated, may either be aleadframe-based or a substrate-based plastic molded housing. With thisinvention, the MEMS may use a standard CMOS chip without any movablestructure and a packaging component with movable structures builttherein.

Applicants further discovered that the separation of movable andelectronics parts provides greater system level integration with othercomponents such as package-on-package MEMS, thus increasing theelectrical product efficiency.

In embodiments, which have the movable elements integrated into asubstrate-based package, the substrate may be a stiff multi-layersubstrate, such a multi-metal-layer FR-4 board, or a flexible filmsubstrate, such as a metalized polyimide tape. The latter devices need amolded encapsulation for robustness. Packages can be stacked with solderbodies as connecting elements.

Embodiments of this invention include the usage of electrostatic force,acceleration, air pressure, etc., to deflect a beam or membrane forbuilding microphones, pressure sensors, accelerometers, and otherapplications where a movable member is needed to convert an externalinput into an electrical output.

Exemplary MEMS of the pressure sensor family, operating on capacitivechanges caused by a movable membrane, may offer 80% lower fabricationcost, when the membrane is integrated into the plastic device packageinstead of being fabricated in conventional manner as a portion of thesilicon chip.

One embodiment of the present invention provides a MEMS comprising: aflat substrate having a thickness, a first surface and an oppositesecond surface; an opening through the thickness of the substrate, theopening extending from the first to the second surface; a metal foilattached onto the first surface of the substrate, the foil including aplurality of pads and a membrane extending at least partially across theopening; and an integrated circuit chip flip-assembled to the pads, thechip at least partially spanning across the opening, separated from themembrane by a gap.

Another embodiment of the present invention provides a method forfabricating a MEMS comprising the steps of: forming an opening from afirst surface to an opposite second surface of a flat substrate;laminating a metal foil onto the first substrate surface and at leastpartially across the opening so that the foil adheres to the substrate;patterning the metal layer into a plurality of pads and a segment; andflip-connecting a semiconductor chip having electronic circuitry ontothe pads so that the chip spans across at least partially across theopening, separated from the segment by a gap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic cross section of an exemplary MEMS of theinertial sensor family operating as a capacitive accelerometer, wherethe movable membrane is part of the substrate-based package. In theexample shown, the substrate is a multi-metal layer insulatingcomposite, and all pads and the membrane are on one surface of thecomposite in the same plane.

FIG. 2 shows a schematic cross section of an exemplary MEMS of theinertial sensor family operating as a capacitive accelerometer, wherethe movable membrane is part of the substrate-based package. In theexample shown, the substrate is an insulating tape and the metal padsand the membrane are on one surface of the tape in the same plane; anencapsulation may be added to enhance the package robustness.

FIG. 3 shows a schematic cross section of an exemplary MEMS of theinertial sensor family operating as a capacitive accelerometer, wherethe movable membrane is part of the substrate-based package. In theexample shown, the substrate is an insulating tape, the membrane andsome pads are on one surface of the tape, other pads on the oppositesurface; an encapsulation may be added to enhance the packagerobustness.

FIG. 4A depicts a schematic cross section of an exemplary MEMS of thepressure sensor family operating in the capacitive mode, where themovable membrane is part of the substrate-based package. In the exampleshown, the substrate is an insulating tape; an encapsulation may beadded to enhance the package robustness.

FIG. 4B shows a top view of an exemplary outline of the membrane in FIG.4A.

FIG. 5A depicts a schematic cross section of another exemplary MEMS ofthe inertial sensor family operating as a capacitive accelerometer,where the movable membrane is part of the substrate-based package.

FIG. 5B is a schematic top view of a membrane for the MEMS in FIG. 5Aincluding lateral sensing.

FIG. 5C is a schematic top view of a fingered membrane for the MEMS inFIG. 5A to increase the sensitivity for lateral movement.

FIG. 5D is a schematic top view of a symmetrically balanced membrane forthe MEMS in FIG. 5A for sensing rotational acceleration.

FIG. 6 shows a schematic cross section of a hermetic MEMS accelerometer,where the electrostatically lifted proof mass is part of thesubstrate-based package.

FIG. 7 depicts a schematic cross section of a hermetic MEMSaccelerometer with a symmetrically balanced proof mass.

FIG. 8 illustrates a schematic cross section of a hermetic MEMSaccelerometer with a semiconductor chip attached on only one side toallow an enlarged proof mass sensor.

FIGS. 9A to 9J illustrate certain process steps of a fabrication flowfor an inertial sensor MEMS with the movable membrane integrated intothe substrate-based package.

FIG. 9A is a schematic cross section of the dielectric substrate withopenings formed form the first to the second substrate surface.

FIG. 9B is a schematic cross section of the substrate after depositing alayer of adhesive material on the first surface of the carrier.

FIG. 9C is a schematic cross section of the substrate after laminating ametal foil across the adhesive layer.

FIG. 9D is a schematic cross section of the substrate after depositing aphotoresist layer across the metal foil and another photoresist layeracross the contoured second substrate surface.

FIG. 9E is a schematic cross section of the substrate after patterningthe metal foil and removing both photoresist layers.

FIG. 9F is a schematic cross section of the substrate after the processstep of flip-connecting semiconductor chips.

FIG. 9G is a schematic cross section of the assembled substrate afterattaching solder balls for external connection.

FIG. 9H shows a schematic cross section of singulated completed inertialsensor MEMS with the movable membrane integrated into thesubstrate-based package.

FIG. 9I is a schematic cross section of the assembled substrate afterdepositing an encapsulation compound.

FIG. 9J shows a schematic cross section of singulated completed inertialsensor MEMS with the movable membrane integrated into thesubstrate-based package strengthened by encapsulation compound.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a schematic cross section of an exemplary embodimentof the invention showing a micro-electro-mechanical system (MEMS) of theinertial sensor family, which operates as a capacitive accelerometerwith displacement-dependent output signals. In these sensors,acceleration is transformed into the displacement of a movable mass orplate; the position change is measured as a change of the capacitancerelative to a fixed plate. Capacitive accelerometers exhibit highsensitivity, good DC response and noise performance, low drift, and lowpower dissipation and temperature sensitivity. The exemplary MEMS,generally designated 100 in FIG. 1, is a system structured like QFN(Quad Flat No-lead) and SON (Small Outline No-Lead) type semiconductordevices.

In the embodiment of FIG. 1, an integrated circuit device, representedby chip 101, is flip-assembled on metallic pads 110. Flip-assemblytypically utilizes conductive spacers such as solder balls or solderbodies to mechanically and electrically attach a chip surface, on whichan integrated circuit has been formed, to an opposing surface of asubstrate which interconnects multiple integrated circuits or otherelectrical components. Metallic pads 110, which are attached to thefirst surface 120 a of a substrate 120. This substrate is flat, has athickness 121, a first surface 120 a and an opposite second surface 120b. Substrate 120 is made of insulating material, which may be apolymeric compound, in some devices strengthened by glass fibers, or aceramic compound, or a glass compound. As examples, substrate 120 may bea sheet or board made of a multi-metal layer composite, as schematicallyindicated in FIG. 1, or it may be a sheet-like multi-metal layer ceramiccomposite. The typical thickness range is between about 70 and 150 μm.Alternatively, the substrate may be a flexible polymeric tape such aspolyimide (as indicated by designation 220 in FIG. 2).

MEMS 100 has an opening 122 through the thickness 121 of substrate 120.In FIG. 1, the opening has a uniform width 122 a; in other embodiments,the width of the opening may not be constant. Openings 122 may be shapedas a cylinder, a truncated cone, or any other suitable stereometricalform. As FIG. 1 shows, opening 122 extends from the first substratesurface 120 a to the second substrate surface 120 b. Flip-assembled chip101 spans at least partially across the opening.

As FIG. 1 illustrates, metallic pad 110 is a portion of a patternedmetal foil attached onto the first surface 120 a of substrate 120; anadhesive layer 130 may be used for the attachment (the processes forattaching, such as laminating, and patterning are described in FIGS. 9Ato 9J). The metal foil may be made of copper or a copper alloy; otheroptions include nickel, or an alloy containing an iron-nickel alloy(such as Alloy 42 or Invar™), or aluminum. Preferred foil thicknessrange is between about 5 and 50 μm, more preferably between 10 and 25μm, but may be thicker or thinner in other embodiments. Other portionsof the patterned metal foil include a plurality of pads 111 forconnection to external parts, often using solder bodies 140, and amembrane 112 as a movable part. FIG. 1 depicts membrane 112 extending atleast partially across the opening 122, and being parallel to chip 101.Membrane 112 is separated from chip 101 by gap 107, which has a height107 a about 10 to 60 μm, typically about 25 μm. Acceleration istransformed into the displacement of the movable membrane, and theposition change is measured as a change of the capacitance relative tothe fixed metal layer 108 on chip 101.

Since pads 110 and 111, and membrane 112 are portions of the metal foilattached to substrate surface 120 a, they have the same thickness 113;for many embodiments, thickness typically is between 10 and 25 μm. Inthis thickness range, membrane 112 is flexible in the direction normalto the first substrate surface and movable in the space of the opening122 and of the gap 107.

The example of FIG. 1 shows a simplified, low cost version, where theopening 122 is not sealed but open; sealed embodiments are illustratedin FIGS. 4A and 5A. In all these embodiments, substrate 120 togetherwith the attached patterned metal foil represent the package for chip101; membrane 112 as the movable part of the MEMS is a portion of thepackage. Examples of the shape of movable part 112 are discussed belowin FIGS. 5A to 8.

In the exemplary embodiment of FIG. 2, the MEMS has a substrate 220 madeof a polymeric tape (for instance polyimide), often in the thicknessrange from about 60 to 100 μm. In this thickness range, substrate 220 isflexible; consequently, in order to have a mechanically robust MEMS,many applications require the substrate to be strengthened by a body 250of hardened plastic compound such as polymerized epoxy-based moldingcompound. The compounds may include inorganic filler particles (such assilicon dioxide or silicon nitride) of about 80 to 90% by volume inorder to better match the coefficient of thermal expansion (CTE) of thecompound to the CTE of silicon. Since the application of theencapsulation compound 250 is optional, it is shown in dashed outline inFIG. 2.

As FIG. 2 shows, tape 220 has an opening 222 through the thickness ofthe tape. In FIG. 2, the opening has a uniform width 222 a; in otherembodiments, the width of the opening may not be constant. In addition,the strengthening body 250 has an opening 252 through the bodythickness. Opening 252 feeds into opening 222. Opening 252 may be shapedas a cylinder, a truncated cone wider on the outside and narrowingtowards width 222 a, as depicted in FIG. 2, or any other suitablestereometrical form. Flip-assembled chip 101 spans at least partiallyacross the opening 222.

The MEMS of FIG. 2 has, in analogy to the device in FIG. 1, a patternedmetallic foil attached to the first surface 220 a of the tape 220. Anadhesive layer 230 may be used for the attachment (the processes forattaching, such as laminating, and patterning are described in FIGS. 9Ato 9J). Typically the metal foil is made of copper or a copper alloy;other options include nickel, or an alloy containing an iron-nickelalloy (such as Alloy 42 or Invar™), or aluminum. The foil thicknessrange is between about 5 and 50 μm, typically between about 10 and 25μm, but may be thicker or thinner in other embodiments. The metal foilincludes an elongated portion 212, which is operable as a movable partor membrane. Other portions of the patterned metal foil include aplurality of pads 211 for connection to external parts, often usingsolder bodies 240, and pads 210 for contacts to semiconductor chip 101.FIG. 2 depicts membrane 212 extending at least partially across theopening 222, and being parallel to chip 201. Membrane 212 is separatedfrom chip 201 by gap 207, which has a height 207 a of about 10 to 60 μm,typically about 25 μm. Acceleration is transformed into the displacementof the movable membrane, and the position change is measured as a changeof the capacitance relative to the fixed metal layer 108 on chip 101.

Since pads 210 and 211, and membrane 212 are portions of the metal foilattached to substrate surface 220 a, they have the same thickness 213;for many embodiments, thickness 213 is between about 5 and 50 μm,typically between 10 and 25 μm, but may be thicker or thinner in otherembodiments. In this thickness range, membrane 212 is flexible in thedirection normal to the first substrate surface and movable in the spaceof the opening 222 and of the gap 207.

Another exemplary embodiment of the invention, generally designated 300,is illustrated in FIG. 3. The embodiment is a substrate-based MEMS ofthe inertial sensor family acting as a capacitive accelerometer withdisplacement-dependent output signals. Substrate 320 is made of apolymeric tape (for instance polyimide), typically in the thicknessrange from about 60 to 100 μm, with first surface 320 a and oppositesecond surface 320 b [note the difference of surface designationscompared to FIGS. 1 and 2]. Tape 320 has a plurality of metal-filled viaholes extending from first surface 320 a to second surface 320 b. Oneset of vias, designated 311, operates as attachment sites for solderballs 340 to external parts, another set of vias, designated 313,operates as sites for flip-attaching chip 101.

Substrate 320 further has an opening 322 through the thickness ofsubstrate 320, extending from first substrate surface 320 a to secondsubstrate surface 320 b. In FIG. 3, the opening has a uniform width 322a; in other embodiments, the width of the opening may not be constant.Openings 122 may be shaped as a cylinder, a truncated cone, or any othersuitable stereometrical form. Flip-assembled chip 101 spans at leastpartially across the opening, forming a gap 307 of height 307 a withsecond substrate surface 320 b. Height 307 a typically is between about10 and 60 μm, often about 25 μm.

A metal foil 310 is attached onto first substrate surface 320 a,typically using an adhesive layer 330 to support the attachment. Themetal foil may be made of copper or a copper alloy; other optionsinclude nickel, or an alloy containing an iron-nickel alloy (such asAlloy 42 or Invar™), or aluminum. The preferred foil thickness isbetween 10 and 25 μm, but may be thicker or thinner in otherembodiments. The foil is patterned to form a membrane 312 without theadhesive layer. In the foil thickness range indicated, membrane 312 isflexible in the direction normal to the first substrate surface 320 aand movable in the space of the opening 322 and of the gap 307.Acceleration is transformed into the displacement of the movablemembrane 312, and the position change is measured as a change of thecapacitance relative to the fixed metal layer 108 on chip 101.

Since the stack of tape 320, adhesive layer 330, and metal foil 310 hasa total thickness in the range from approximately 70 to 150 μm, it isflexible. When some applications require a mechanically more robustMEMS, the stack may be strengthened by adding a body 350 of hardenedplastic compound such as a polymerized epoxy-based molding compound,optionally filled with inorganic particles of silicon dioxide or siliconnitride. The strengthening body 350 has an opening 352 through the bodythickness. Opening 352 feeds into opening 322 so that is allows theunobstructed operation of membrane 312 in moving in the z-direction.Opening 352 may be shaped as a cylinder, a truncated cone wider on theoutside and narrowing towards width 322 a, as depicted in FIG. 3, or anyother suitable stereometrical form.

Flip-assembled chip 101 spans at least partially across the opening 322so that metal plate 108 forms a capacitor with membrane 312. Plate 108is separated from membrane 312 by a distance, which is composed of thesum of gap height 307 a, the thickness of tape 320, and the thickness ofadhesive layer 330. As stated above, the movable part, membrane 312, canmove in this distance in the z-direction, normal to the plane of themembrane. In some embodiments, movable part 312 includes the suspensionbeam of length 315 and the movable plate of length 316. Movable plate316 has an area equal to the area of the fixed plate 108 on the chipsurface in order to form a capacitor. In addition, for some embodimentsthe mass of the movable plate 316 can be enlarged by adding the mass ofa deformed gold sphere 314, as formed in the well-known wire ball bondprocess. Mass 314 represents the proof mass.

The invention allows the selection of the materials and dimensions foropening 322 (and 352), length of suspension beam 315, area of movableplate 316, mass 314, and capacitance between movable plate 316 and fixedplate 108. Consequently, the accelerometer of FIG. 3 can be specializednot only as a capacitive displacement sensing accelerometer, whichtransforms acceleration into the displacement of a movable mass, butalso as a force sensing accelerometer, which detects directly the forceapplied on a proof mass. The mechanical transfer function of theselected components relates applied acceleration as the input to thedisplacement of the mass (movable plate 312 and mass 314) as the output.The components of FIG. 3 allow a designed distribution of the outputbetween the additive forces: inertial force, elastic force, and dampingforce.

The exemplary embodiments shown in FIGS. 4A, 5A, 6, 7, and 8 illustrateMEMS with enclosures, which are quasi-hermetic in polymeric packages,and fully hermetic in packages using ceramic or glassy encapsulations.The embodiment of FIG. 4A displays a QFN/SON-type MEMS, generallydesignated 400, of the pressure sensor family, which operates in thecapacitive mode with displacement-dependent output signals. Anintegrated circuit chip 401 is flip-assembled onto metallic pads on thesurface of a substrate 420. The substrate may be a multi-metal layerinsulating composite, as discussed above in FIGS. 1, or a polymericsheet, as discussed in FIG. 2, or a polymeric sheet with metal-filledvia holes, as discussed in FIG. 3. Substrate 420 in FIG. 4A illustratesthe latter embodiment. The metal-filled vias are designated 421. The useof solder balls 440 for interconnection makes these devices especiallysuitable for high input-output counts, multi-chip modules andpackage-on-package modules.

Substrate 420 has an opening 422 through the thickness of the substrate,extending from first substrate surface 420 a to second substrate surface420 b. In FIG. 4A, the opening has a uniform width; in otherembodiments, the width of the opening may not be constant. The openingsmay be shaped as a cylinder, a truncated cone, or any other suitablestereometrical form. The flip-assembled chip 401 spans at leastpartially across the opening, forming a gap 407 with first substratesurface 420 a. The gap has a typical height between about 10 and 60 μm,more typically about 25 μm.

Substrate 420 has a patterned metal foil attached to its first substrate420 a; an optional adhesive attachment layer is not shown in FIG. 4A.The foil typically is made of metal such as copper or nickel,alternatively of an iron-nickel alloy (such as Alloy 42 or Invar™) or ofaluminum. For many embodiments, the thickness is between about 5 and 50μm, preferably between about 10 and 25 μm, but may be thicker or thinnerin other embodiments. The pattern of the foil includes at least onemovable part and a plurality of pads; in general, the pattern allowscomplicated routings of signals. In the thickness range quoted, themovable part 412 can act as a membrane, which is flexible in thez-direction, movable in the space of the opening 422 and of the gap 407.As a membrane, part 412 is sensitive to external pressure changesarriving from z-direction through opening 422, bending the membraneinward and outward of gap 407. In some embodiments, movable part 412 hasan area between about 0.5 and 2.3 mm²; in other embodiments, the areamay be smaller or larger. In some MEMS, the membrane may be divided in amovable plate 412 a and suspension beams 412 b, which hold the plate.Plate 412 a may have a rectangular or a rounded outline; an example isillustrated in the top view of FIG. 4B. The suspension beams 412 b maytake a wide variety of configurations (angular, spring-like, rounded,etc.) to enhance the pressure sensitivity; FIG. 4B depicts an angularconfiguration. The movable membrane 412 is facing metal plate 408 onchip 401 to form a capacitor across gap 407.

For the exemplary embodiment of FIG. 4A, the plurality of pads may begrouped in sets. The pads of the first set, designated 410, enableelectrical interconnection between the movable part 412 and theintegrated circuit of chip 401. The leads of the second set, designated413, enable contacts to external parts; they allow the attachment ofsolder balls 440. The leads of the third set, designated 411, areconfigured as a metal seal ring encircling the opening 422. In general,patterning of the foil enables high density interconnects as well ascomplicated routing of signals.

For the substrate pad sets 410 and 413, a plurality of chip terminals402 allow the connections to solder bodies, gold bumps, or gold alloy.The gold bumps may be produced by a wire ball bonding technique,followed by a flattening process with a coining technique. The goldalloy may be a low melting gold/germanium eutectic with 12.5 weight % Geand an eutectic temperature of 361° C. In addition, chip terminal 403may be configured as a seal ring to allow the formation of a seal ringmade of solder or gold alloy to seal the enclosed space at leastquasi-hermetically, e.g. against environmental disturbances such asparticles, but not completely against gaseous and moisture molecules.FIG. 4A shows the optional encapsulation 450 material filling the spacebetween the chip 401 and substrate 420 up to the seal ring between pad411 and chip terminal 403. The encapsulation 450 typically is fabricatedby a molding technique (for instance transfer molding) using anepoxy-based molding compound; the compound is hardened by polymerizationto give mechanical strength to device 400. The compound may includeinorganic filler particles (such as silicon dioxide or silicon nitride)of about 80 to 90 volume % in order to lower the coefficient of thermalexpansion (CTE) close to the silicon CTE.

Sensing plate 408 and membrane 412, typically having the same area andbeing separated by a gap, form a capacitor. As stated above, membrane412 is made of a metal (for example, copper) thin enough (for example,10 μm) to be flexible and sensitive to pressure changes. The assembleddevice 400, therefore, works as a MEMS for pressure sensor andmicrophone. Responding to pressure arriving through opening 422 bybending inward and outward, membrane 412 modifies distance 407 relativeto stationary plate 408. Let the area of membrane 412, and plate 408, aselectrodes be A; the distance between the electrodes under originalpressure be D_(o); and the dielectric constant of the space between theelectrodes be E, then the capacitance C of the electrodes is given byC=∈·A/D _(o).

Pressure in z-direction deforms the flexible membrane so that thedeformed area has to be calculated as an integral over small areaelements dx·dy, while the distance D_(o) is modified in both x-directionand y-direction by a deflection w_(x,y). The resulting change ofcapacitance is measured by the circuitry of chip 401, operating as amicrophone or a pressure sensor. A miniature speaker can be built in asimilar way by driving the membrane electrostatically.

It should be stressed that the embodiments shown in FIGS. 1, 2, 3, and4A lend themselves to stacking of devices; these stacked devices includemulti-chip MEMS as well as package-on-package MEMS.

In order to give a cost estimate for the exemplary pressure sensor MEMS,the side lengths of the molded material 450 in FIG. 4A may be 3 by 3 mm,4 by 4 mm, 3 by 4 mm, or any other size desired by customers. The basematerial of the substrate may be polyimide, and the metal foil,including the membrane 412, may be copper. The cost of the moldedpackage, including the movable part, in mass production is about $0.13.With the added cost of the chip about $0.009, the total cost of the MEMSin a plastic package including the movable part according to theinvention is about $0.139. This cost compares to the cost of aconventional pressure sensor MEMS of the same body sizes and a FR-4based substrate material as follows: The cost of the conventionalpackage is about $0.54; the cost of the chip including the movable partis about $0.017; the total cost of the MEMS about $0.557. This cost isapproximately four-fold the cost of the MEMS according to the invention.

FIGS. 5A to 5D illustrate a MEMS embodiment as a variable capacitor andRF switch, with the possibility for a fully hermetic package, suitablefor cellular handset antenna tuning. The construction of the exemplaryMEMS in FIG. 5A is similar to the device described in FIG. 1 with thefollowing additions. The opening 522 of substrate 520 (shown as amulti-level metal insulating compound or ceramic) is sealed by a lid 550in flat contact with substrate 520. The lid material may be plastic forquasi-hermetic sealing, or, if fully hermetic sealing is needed for aceramic substrate, a moisture-impenetrable material such as glass,ceramic, or, in some devices, metal. Substrate pad 511 and contact 503of chip 501 are configured as a seal rings to allow complete sealing bysolder or a low-melting gold alloy. Membrane 512 as the movable partextends at least partially across opening 522. Membrane 512 is shown inFIG. 5A as membrane-at-rest in solid contour, and as deflected membranein dashed contour. When deflected, the membrane rests on dielectric film508 a attached to chip plate 508. In this position, the membrane forms ahigh capacitance with the plate, representing a low impedance for RFfrequencies; the RF switch is turned on.

The concept to integrate the movable part of a MEMS with asubstrate-based package rather than with the chip allows numerousvariations with the goal to sensitize certain aspects of themeasurement, or to include new aspects into the measurement. FIGS. 5B,5C, 5D, 6, 7, and 8 highlight only a few select variations andpossibilities and are not intended to be construed in a limiting sense.The figures should emphasize the great number of possibilities of theinvention apparent to persons skilled in the art.

FIGS. 5B, 5C. and 5D show some examples of membrane configurations inleaf spring patterns in order to reduce electrostatic forcerequirements, or to add lateral movement sensitivity. In FIG. 5B, themembrane is divided into a moving portion 512 a and a fixed portion 512b; the design helps to minimize the risk of membrane sticking to theplate on the chip. In FIG. 5C, the design of fingered membranes 512 c,512 d, and 512 e surrounded by fixed metal portions 560 increaseslateral motion and the sensitivity to lateral movements of the membrane;the lateral movement is detected by capacitance measurement). FIG. 5Dshows that the enhanced designs can also be made both fingered andbalanced. The lateral sensing electrodes 512 f are symmetricallyarranged for sensing rotational acceleration (indicated by arrows 570).The quoted structures may adjust proof mass, for instance by addingsquashed balls from wire bonding, see FIG. 3), and spring constant sothat they can be tuned for different applications.

FIGS. 6, 7, and 8 show additional examples of MEMS accelerometers toillustrate the variety of design options for devices with single membermembranes integrated into substrate-based (620, 720, 820) packages withattached metal foil (610, 710, 810). The MEMS are encapsulated bymolding compound (650, 750) in FIGS. 6 and 7, or by a can-type housing(850) in FIG. 8. The single member membrane design is depicted in FIG. 6in zero position (612 a) and activated (hovering) position (612 b). Bylifting the proof mass from the substrate surface using electrostaticforce from the plate on the chip above the substrate, the lateralelectrodes can also be made to electrostatically hover to detectvertical motion by restoring force signals; the force needed to keep theproof mass in place is proportional to the acceleration. Hoveringmembranes may be passivated when they are made of a corrosive metal suchas copper.

The symmetrically balanced design of a single membrane is shown in FIG.7 in activated (hovering) position (712 b). As stated above, theaddition of lateral sensing electrodes allows the sensing of lateralmovements of the membrane by capacitance measurements.

FIG. 8 illustrates a MEMS with mounting chip 801 on one side only sothat the smaller chip allows a membrane 812 with a sensor enlarged byproof mass 814. The sensitivity of the accelerometer is enhancedcompared to the arrangement discussed in FIG. 6. In general, MEMS withthe movable element integrated into substrate packages such as shown inFIGS. 5A, 6, 7, and 8 lends themselves to multi-chip and stackedpackage-on-package applications.

Another embodiment of the invention is a process for fabricatingsubstrate based MEMS with the movable element integrated into the devicepackages. For the process flow shown in FIGS. 9A to 9J, the processstarts in FIG. 9A by selecting a flat substrate 920. The substrate maybe a sheet or board of a multi-metal layer plastic composite, amulti-metal layer ceramic composite, or a glass-fiber strengthened boardsuch as FR-4, with an exemplary thickness between 70 and 150 μm.Alternatively, substrate 920 may be a polymeric tape such as apolyimide-based tape. The substrate has a first surface 920 a and anopposite second surface 920 b. In the next process step, openings 922are formed through the thickness of the substrate, from the first to thesecond surface. One method of forming the openings is a punchingtechnique.

In FIG. 9B, a layer 930 of an adhesive material is attached to thesubstrate across the first surface 920 a, including across the openings922. The adhesive material is selected to stick to the substrate as wellas to metal. In FIG. 9C, a metal foil 910 is laminated across theadhesive layer on the first substrate surface. Suitable metals includecopper, nickel, and aluminum, and the thickness range typically is fromabout 10 to 25 μm, more generally from about 5 to 50 μm. The thicknessof the metal foil is selected so that the metal has the flexibility tooperate as the membrane of the MEMS.

The patterning of metal layer 910 into a plurality of pads and asegment-to-become-membrane begins with depositing a photoresist layer960 across the metal foil on the flat first substrate surface andanother photoresist layer 961 across the contoured second substratesurface. As FIG. 9D shows, photoresist layer 961 follows the outline ofthe openings 922, where it also adheres to adhesive layer 930. Whilephotoresist layer 960 is masked, developed, and etched, photoresistlayer 961 remains unaffected. When metal layer 910 is etched andpatterned, photoresist layer 961 protects substrate surface 920 b.Finally, when photoresist layer 960 is removed, photoresist layer 961 isalso removed, and with it the portions of adhesive film 930, onto whichphotoresist layer 961 had adhered.

The result of the patterning steps for metal layer 910 is illustrated inFIG. 9E. The patterning created segment 912, which is to become themoving part of the MEMS, for instance the membrane, and the plurality ofpads 911. As membrane, segment 912 extends at least partially acrossopening 922. As a further result, the patterning of metal foil 910creates interconnecting traces not shown in FIG. 9E.

In the step depicted in FIG. 9F, a semiconductor chip 901 withelectronic circuitry is flip-connected onto some of the metal pads sothat the chip extends at least partially across membrane 912 and opening922. The chip attachment is facilitated by bumps 970 made of solder orof gold. After the chip attachment, a gap 907 is formed between chip 901and membrane 912. Gap 907 is mostly determined by the height of bumps970.

In the next process step, illustrated in FIG. 9G, connecting metalbodies 940, such as solder balls, solder paste, or bodies of alow-melting gold alloy, are attached to a plurality of pads 911. Next,substrate 920 is prepared for the cuts along line 980, for instance by asaw, to singulate the substrate into discrete MEMS devices. Theseseparated units are depicted in FIG. 9H; each unit is an exemplary MEMSas described in FIG. 1.

Alternatively, before the cuts along line 980 in FIG. 9G areadministered, the process step shown in FIG. 9I may be performed. Theun-singulated substrate with the assembled chips is transformed into amore robust configuration by an encapsulation technology. In one processembodiment, the substrate with the assembled chips and the solder ballsattached is loaded into a mold for a transfer molding process. Theencapsulation material 950 is deposited over the features on substrateside 920 a so that cavity 922 remains open, or may, in some MEMS,actually widen, as suggested in FIG. 9I. The encapsulation process maybe a transfer molding technique, and the encapsulation material may bean epoxy-based polymeric molding compound selected so that the compoundadheres strongly to substrate 920. As stated above, steel hillocksprotruding into the mold cavity used for transfer molding technologyoffer a low cost way to prevent a filling of opening 822 with compound.After the molding step, the polymeric compound 950 is hardened bypolymerization, resulting in a sturdy package for the MEMS.

Next, substrate 920 is prepared for the cuts along line 980, forinstance by a saw, to singulate the substrate into discrete MEMSdevices. These separated units are depicted in FIG. 9J; each unit issimilar to the exemplary MEMS described in FIG. 2.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. As an example, the invention applies to any material forthe MEMS package, including plastics and ceramics, and the semiconductordevice, integrated circuits as well as discrete devices, includingsilicon, silicon germanium, gallium arsenide, or any other semiconductoror compound material used in manufacturing.

As another example, the integration of the movable element into theleadframe-based package of a MEMS can be applied to piezoresistivepressure sensors, where the conversion of pressure to an electronicallydetectable signal relies on the elastic deformation of a membrane, orgenerally of a structure, that is exposed to the pressure.

As another example, the integration of the movable element into theleadframe-based package of a MEMS can be applied to resonant pressuresensors, where the resonance frequency depends on the mechanical stressin the vibrating microstructure.

As another example, the method of integrating the movable element intothe MEMS package allows an inexpensive fine-tuning of the mechanicaltransfer function by controlling the thickness of the membrane and byadding one or more mass units of squashed balls produced in wire bondingtechnique.

As another example, the integration of the movable element into thesubstrate-based package of a MEMS can be applied to piezoresistivepressure sensors, where the conversion of pressure to an electronicallydetectable signal relies on the elastic deformation of a membrane, orgenerally of a structure, that is exposed to the pressure.

As another example, the integration of the movable element into thesubstrate-based package of a MEMS can be applied to resonant pressuresensors, where the resonance frequency depends on the mechanical stressin the vibrating microstructure.

As another example, the method of integrating the movable element intothe substrate-based MEMS package allows an inexpensive fine-tuning ofthe mechanical transfer function by controlling the thickness of themembrane and by adding one or more mass units of squashed balls producedin wire bonding technique.

It is therefore intended that the appended claims encompass any suchmodifications or embodiments.

1. A micro-electro-mechanical system (MEMS) comprising: a flat substratehaving a thickness, a first surface, an opposite second surface, and anopening through the thickness of the substrate, the opening extendingfrom the first to the second surface; a metal foil attached onto thefirst surface of the substrate, the foil including a plurality of padsand a membrane extending at least partially across the opening; and anintegrated circuit chip flip-assembled to the pads, the chip at leastpartially spanning across the opening, separated from the membrane by agap.
 2. The MEMS of claim 1 wherein the metal foil is attached by anadhesive onto the surface of the substrate.
 3. The MEMS of claim 2further including metal-filled via holes through the thickness of thesubstrate.
 4. The MEMS of claim 1 wherein the substrate is a sheet-likemulti-metal-layer plastic composite.
 5. The MEMS of claim 1 wherein thesubstrate is a sheet-like multi-metal-layer ceramic composite.
 6. TheMEMS of claim 1 wherein the substrate is a polymeric tape.
 7. The MEMSof claim 1 wherein the membrane is movable normal to the first surfacein the space of the opening and of the gap.
 8. The MEMS of claim 1wherein the membrane seals the opening at the first substrate surface.9. The MEMS of claim 1 wherein the membrane has the configuration of asuspension beam and a plate movable normal to the first substratesurface.
 10. The MEMS of claim 9 further including an additional massattached to the plate.
 11. The MEMS of claim 1 wherein the metal padsinclude a first set and a second set, the pads of the first set enablingelectrical interconnection between the membrane and the integratedcircuit, the pads of the second set enabling contacts to external parts.12. The MEMS of claim 11 wherein the metal pads further include a thirdset configured as a metal seal ring encircling the opening.
 13. The MEMSof claim 1 further including a metal plate on the chip, the plate facingthe first substrate surface and forming a capacitor with the membrane.14. A method for fabricating a micro-electro-mechanical system (MEMS)comprising the steps of: forming an opening from a first surface to anopposite second surface of a flat substrate; laminating a metal foilonto the first substrate surface and at least partially across theopening so that the foil adheres to the substrate; patterning the metallayer into a plurality of pads and a segment; and flip-connecting asemiconductor chip having electronic circuitry onto the pads so that thechip spans at least partially across the opening, separated from thesegment by a gap.
 15. The method of claim 14 further including, beforethe step of laminating, the step of attaching a layer of adhesivematerial across the first surface of the substrate, including across theopening, the material being sticky to the substrate and to the metalfoil.
 16. The method of claim 14 wherein the metal foil has a thicknesssuitable for membranes of MEMS.
 17. The method of claim 14 furtherincluding the step of forming the segment so that it extends at leastpartially across the opening and is operable as a membrane.
 18. Themethod of claim 14 further including the step of strengthening theassembled substrate by depositing and hardening a polymeric compoundwhile preserving the opening in the substrate.